150 On the Limiting Thickness of Liquid Films. [Apr. 19, 



have entailed considerable alterations in details, but the main features 

 of the apparatus remain unaltered. The new form, however, 

 possesses the important advantage that the temperature and hygro- 

 metric state of the air in contact with the films can be kept perfectly 

 constant during the progress of the experiments. With this ap- 

 paratus a number of measures have been made of the electrical 

 resistance of films which have thinned sufficiently to show the black 

 of the first order of Newton's rings. The chief interest of these lies 

 in the information which they afford as to the thickness of such films. 

 To deduce the thickness from the resistance, it is necessary to assume 

 that the specific resistance of the films is the same as that of the liquid 

 in mass. The authors' previous experiments do not enable them to 

 assert the truth of this assumption for such thin films, and it was 

 therefore important to ascertain by an independent method whether it 

 might be taken as approximately true. 



For this purpose between 50 and 60 plane films were formed in a 

 glass tube 400 mm. long, and 18 mm. in internal diameter. The 

 tube was closed by pieces of plate glass, and placed in the path of 

 one of the interfering rays in a Jamin's " interferential refractometer." 

 When the films had become black, a known number were broken by 

 bringing an electromagnet near to the tube, and thus moving some 

 sewing needles, which had been enclosed along with the films. The 

 mean thickness of the films was deduced from the displacement of the 

 interference " fringes " caused by their rupture. For reasons given in 

 the paper two tubes were used. One was placed in the path of each 

 of the interfering rays, and the mean of the values obtained by 

 breaking the films inoach tube in turn was taken as the result of the 

 experiment. 



Two liquids were observed, viz., M. Plateau's " liquide glycerique," 

 and a soap solution containing no glycerine. Details are given in the 

 paper. The following are the means of the various groups of 

 observations. 



Liquid. 



Method. 



Mean thickness in terms 

 of 10~ 6 millims. 



" Liquide glycerique " . . 



Electrical 



11 9 

 10-7 



11 -7 



12 1 



The agreement between these numbers is sufficiently close to make 

 the fact that they are approximately correct unquestionable, and to 

 prove that the mean thickness of a black film is nearly the same for 

 both liquids. 



